| sorenj07 |
Hi, first post in this neck of the woods :) I've got a 2x30V 300VA Hammond toroid kicking around. Would it be suitable for building a gainclone using the standard LM3875T chips? I usually see secondaries around 18-22V being used. I would probably have a rail voltage of around +- 42V. On the other hand, I could wire the dual primaries in series to put out +-21V or so, but this seems a little low... Speakers used would be 8 ohms.
Does anyone happen to know the input impedance of the LM3875 by the way? It'd be useful in designing the cathode follower which would feed the volume control. |
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| sorenj07 |
| Am I right in assuming, then, that 42V will be ok into 8 ohms since it's well under the maximum rating of 80V or so? I'm new to these "solid state" things :smash: I assume I should try to mount them to a heatsink, right? |
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| ionomolo |
| quote: | | Originally posted by sorenj07Am I right in assuming, then, that 42V will be ok into 8 ohms since it's well under the maximum rating of 80V or so? I'm new to these "solid state" things I assume I should try to mount them to a heatsink, right? |
Yes, you need a big one! there is an application note on heatsinking national amplifiers and some information on the datasheet. |
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| sorenj07 |
| All right. Do you think some CPU style heatsinks with a mounted fan would work all right? I have no clue about dissipation ratings and the like, but I will take another look at the datasheet. It's just so long :eek: |
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| Nordic |
| quote: | Originally posted by sorenj07
All right. Do you think some CPU style heatsinks with a mounted fan would work all right? I have no clue about dissipation ratings and the like, but I will take another look at the datasheet. It's just so long :eek: |
Not a chance.... They dissipate about 60W with fans blowing like mad. If at all possible issolate the heatsink from the case, then mount the chip useing only a little thermal grease... but you are going to need a much sturdier sink than that... |
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| sorenj07 |
There are some interesting-looking options for the tweaker market..
http://www.newegg.com/Product/Produ...&page=1&bop=And
Either some of the more crazy ones, or I start looking for massive blocks. Pity, I figured that these chips wouldn't need that much sinking. |
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| DC Dave |
| quote: | Originally posted by sorenj07
Am I right in assuming, then, that 42V will be ok into 8 ohms since it's well under the maximum rating of 80V or so? I'm new to these "solid state" things :smash: I assume I should try to mount them to a heatsink, right? |
Wait a minute.
That is going to be to high. 42+42=84V. The 80V maximum referred to is the sum of the + and -. You are going to want a lower voltage for that chip. |
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| sorenj07 |
Hey, looks perfect!
What do you make of using the following schem for the signal part?
http://www.geocities.com/rjm003.geo...dio/diy_gc.html
specifically, this:
Or should I just use the textbook example from the datasheet, under "Typical Application"? Anyone have any recommendations? |
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| ionomolo |
| I would go for the unregulated one since the LM3886 has a good psrr and some regulated power supplies have problems at high frequencies, but there is people here reporting an improvement in sound as a consequence of using a regulated supply. There are schematics of discrete regulators floating arround that may suit your needs if you want it complicated. |
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